The following patents and publications are believed to represent the current state of the art:
U.S. Pat. Nos.:
    5,969,855; 5,969,853; 5,936,253; 5,870,191; 5,814,815; 5,751,475; 5,619,372; 5,600,440; 5,471,303; 5,446,540; 5,235,587; 4,407,569; 4,190,366;Non-U.S. patents:    JP 9230247 (Abstract); JP 9179029 (Abstract); JP 8094936 (Abstract); JP 7261089 (Abstract); JP 7225341 (Abstract); JP 6186504 (Abstract);Other Publications:    Phillion D. W. “General methods for generating phase-shifting interferometry algorithms”—Applied Optics, Vol. 36, 8098 (1997).    Pluta M. “Stray-light problem in phase contrast microscopy or toward highly sensitive phase contrast devices: a review”—Optical Engineering, Vol. 32, 3199 (1993).    Noda T. and Kawata S. “Separation of phase and absorption images in phase-contrast microscopy”—Journal of the Optical Society of America A, Vol. 9., 924 (1992).    Creath K. “Phase measurement interferometry techniques”—Progress in Optics XXVI, 348 (1988).    Greivenkamp J. E. “Generalized data reduction for heterodyne interferometry”—Optical Engineering, Vol. 23, 350 (1984).    Morgan C. J. “Least-squares estimation in phase-measurement interferometry”—Optics Letters, Vol. 7, 368 (1982).    Golden L. J. “Zernike test. 1: Analytical aspects”—Applied Optics, Vol. 16, 205 (1977).    Bruning J. H. et al. “Digital wavefront measuring interferometer for testing optical surfaces and lenses”—Applied Optics, Vol. 13, 2693 (1974).